Much of the interface provided by Shiny is based on the html elements, styling, and javascript provided by the Bootstrap library.
Knowing the specifics of html (and Bootstrap specifically) are not needed for working with Shiny -but understanding some of its conventions goes a long way to helping you customise the elements of your app (via custom CSS and other tools).
This is not the only place that Bootstrap shows up in the R ecosystem - both RMarkdown and Quarto html documents use Bootstrap for styling as well.
bslib
The bslib R package provides a modern UI toolkit for Shiny, R Markdown, and Quarto based on Bootstrap.
It provides,
Custom theming of Shiny apps and R Markdown documents
Switch between different versions of Bootstrap
Modern UI components like cards, value boxes, sidebars, and more.
This last set of features is what we will focus on now, with more on the first two in next module.
Cards
Cards are a UI element that you will recognize from many modern websites.
They are rectangular containers with borders and padding that are used to group related information.
When utilised properly to group related information, they help users better digest, engage, and navigate through content